switch the website to mdbook

[simonmichael]

"This is exploratory, and in a separate branch because I couldn't make mdbook work nicely with sphinx, or with source files in the root dir. mdbook is simpler and faster and has some nice features."

mdbook is fragile and buggy, but also much simpler and faster, and producing better results in some ways, than our sphinx setup. I think I'll be going ahead with this, probably synced with the next hledger release. The WIP site can be viewed at https://test.hledger.org. The biggest issue remaining is how to serve the old manual versions. Since I don't want them inflating the sidebar or search results, and mdbook can't yet render them without that, they'll probably be rendered and saved outside the main site somehow.

Quick list of improvements for users:

• sidebar can be resized and hidden
• sidebar shows just a list of pages; topics within pages now appear as a separate TOC on the right (for long pages). This is perhaps less elegant but I think easier to use, especially on phones.
• colour theme can be changed
• search gives results as you type
• old manuals are no longer part of the main site, and don't clutter search results
• the default manuals shown are for the current release, not the master branch
• you can view the whole site as a single document (or print, but it's 600 letter pages)

[simonmichael]

hledger.org is now generated by mdbook. Next I'll be adding the version selector to the user manuals (until then you can manually add /VER to the url). Any other breakages will be fixed eventually, or sooner with your help. The sphinx site is visible at http://old.hledger.org for comparison. Thanks!